An Advent calendar is a special calendar used to count or celebrate the days in anticipation of Christmas. The days always overlap with the Christian season of Advent. Despite the name, most commercially available Advent calendars begin on December 1, regardless of when Advent begins, which can be as early as November 27 and as late as December 3. Many take the form of a large rectangular card with "windows" of which there are usually 24: one for each day of December leading up to Christmas Day. The doors are opened starting with the first one. Consecutive doors are opened every day leading up to Christmas.